Multimodel-Based Clear-Air Turbulence Forecast Using Improved Ellrod−Knox Index

Clear Air Turbulence (CAT) is one of the most hazardous en-route weather phenomena in the aviation industry. It is not well detected by on-board radar, and normally occurs in good weather condition without any convective clouds, which makes it more difficult to forecast. One of the well-known areas for CAT encounters is near upper-level jets/frontal system in which strong vertical wind shear (VWS) generates Kelvin-Helmholtz instability to directly affect cruising aircraft, while it cascades down to smaller scale. CAT also occurs via emission and local breakdown of Inertia Gravity Waves (IGWs) near the exit region of the jet stream where strong divergent and geostrophically unbalanced (ageostrophic) flows are dominant. Anticyclonic curvature and shear regions of upper-level ridges are also vulnerable for emission of the IGWs and inertial instability for CAT encounters via geostrophic adjustment.

For strategic flight planning (06-36 hr forecasts), two World Area Forecast Centers (WAFCs) in London and Washington have been using Numerical Weather Prediction (NWP) model output to diagnose areas with a high potential of CAT encounters by the Ellrod index, which is a simplified version of Miller’s frontogenesis function that combines VWS and total horizontal deformation (DEF). In WAFC Washington [NOAA/Aviation Weather Center (AWC)], it has been updated to include a divergence tendency term (DVT), called Ellrod-Knox index, to capture more CAT potential regions associated with the IGWs. Recently, it has been verified as the best single diagnostic among the various CAT diagnostics in the Graphical Turbulence Guidance (GTG) system.

In this study, we updated the Ellrod-Knox index to make it more useful for global CAT forecasts in WAFC Washington (NOAA/AWC) by adding the absolute values of the DVT in the formulation rather than just positive DVT values. Physically, this is supported strongly that any regions having large temporal variance of divergence near the jet stream can emit the IGWs, which in turn generates CAT. To make it a more consistent product and take into account the uncertainty of weather forecast, we applied the updated Ellrod-Knox index to the multi-model outputs from The International Grand Global Ensemble (TIGGE) database. Eight global NWP model outputs on the upscaled 0.5 x 0.5 degree domain with 18-hr daily forecasts issued at 00 UTC at 200, 250, and 300 hPa levels are archived for 6-month from Oct 2015 to Mar 2016. We calculate the Ellrod-Knox index at 250 hPa level (about 34,000 ft and 10.4 km Above the Ground Level), which is a conventional cruising altitude for long-haul commercial aircraft. Preliminary case studies show that the improved Ellrod-Knox index gives better performance than old one against observed Pilot Reports (PIREPs). Detailed comparisons and statistical evaluations using longer-term period of the in situ turbulence observations like Eddy Dissipation Rate (EDR) and Derived Equivalent Vertical Gust (DEVG) will be presented in the conference.